In a moving coil galvanometer a radial magnetic field is obtained with concave magnetic poles, to have

Options

A. uniform magnetic field
B. the plane of coil is parallel to the field in any orientation of coil
C. a non-linear scale for galvanometer
D. both 1 and 2

Answer

To solve the question regarding the moving coil galvanometer and the role of concave magnetic poles in obtaining a radial magnetic field, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding the Moving Coil Galvanometer**:
- A moving coil galvanometer is an instrument used to measure small electric currents. It consists of a coil of wire suspended in a magnetic field.

2. **Role of Concave Magnetic Poles**:
- The question states that concave magnetic poles are used to create a radial magnetic field. This is important because a radial magnetic field ensures that the magnetic field strength is uniform across the area of the coil.

3. **Uniform Magnetic Field**:
- A uniform magnetic field is essential for the galvanometer to function correctly. With a uniform field, the torque acting on the coil remains constant, which allows for accurate measurements. The concave shape of the poles helps in achieving this uniformity.

4. **Torque on the Coil**:
- The torque (\( \tau \)) acting on the coil can be expressed as:
\[ \tau = n \cdot i \cdot A \cdot B \cdot \sin(\theta) \]
where:
- \( n \) = number of turns in the coil
- \( i \) = current through the coil
- \( A \) = area of the coil
- \( B \) = magnetic field strength
- \( \theta \) = angle between the magnetic field and the normal to the coil's plane.

5. **Condition for Maximum Torque**:
- For the torque to be maximum and constant, \( \sin(\theta) \) should be equal to 1, which occurs when \( \theta = 90^\circ \). This means that the area vector of the coil must be perpendicular to the magnetic field.

6. **Plane of the Coil Parallel to the Magnetic Field**:
- To maintain \( \theta = 90^\circ \) consistently, the plane of the coil must remain parallel to the magnetic field at all times. The concave magnetic poles help in achieving this alignment, ensuring that the area vector of the coil remains perpendicular to the magnetic field.

7. **Conclusion**:
- Therefore, the use of concave magnetic poles in a moving coil galvanometer serves two main purposes:
- To create a uniform magnetic field.
- To ensure that the plane of the coil is parallel to the magnetic field in any orientation of the coil.

8. **Final Answer**:
- The correct option is D: both 1 (uniform magnetic field) and 2 (to have the plane of coil parallel to the field in any orientation).